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Stock-Based Compensation - Plan Information (Details) - 2012 Plan - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Stock based compensation        
Number of awards approved for grant (in shares) 4,000,000   4,000,000  
Common stock remaining available for issuance (in shares) 680,623   680,623  
Compensation expense $ 2.5 $ 2.5 $ 5.0 $ 4.8
Board of Directors        
Stock based compensation        
Common shares granted to members of board of directors as compensation for director services     14,895 10,024